[swift-evolution] Proposal: Contiguous Variables (A.K.A. Fixed Sized Array Type)

[Justin Kolb]

I’m not sure I like the fact that they would be associated with tuples. As far as I’m aware tuples don’t have a public or well documented memory layout and most likely it would be better to keep them that way. To be compatible with C a fixed size array would have to have similar memory layout requirements. Also I assume there was a reason not to allow subscripting of tuples originally and adding it to something similar might confuse things. If any of these things are non-issues then I’m fine with them looking like tuples. I do think they will need to be able to be initialized with array like initializers in some form.

I somewhat wish they would have been added in originally as it seems like it will be trickier to find a good form that isn’t already taken. I think it would probably be problematic for the parser to make use of `*` or `x`. Maybe after moving the count to the front of the declaration `:` would end up being different enough from other uses of `:` to make this work:

let values: (4:Int)


> On Jan 28, 2016, at 9:04 PM, Trent Nadeau <tanadeau at gmail.com> wrote:
> 
> That looks great except that I would prefer (4 * Int). Having "x" as an operator looks very odd to me, and it doesn't fit with the rest of the language.
> 
> On Thu, Jan 28, 2016 at 8:07 PM, Erica Sadun via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> I find 
> 
> let values: (4 x Int) = (1, 2, 3, 4)
> 
> to be adequately cromulent. I believe this approach to be:
> 
> * Readable, even to someone unfamiliar with the syntax
> * The parens before the assignment suggest something to do with tuples, and the numbers match the arity after the assignment
> * The type is preserved in-place
> * It's compact, elegant, simple
> 
> -- E
> 
> 
>> On Jan 28, 2016, at 5:56 PM, Joe Groff via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> 
>>> On Jan 28, 2016, at 4:04 PM, Haravikk <e-mail at haravikk.me <mailto:e-mail at haravikk.me>> wrote:
>>> 
>>> 
>>>> On 28 Jan 2016, at 22:37, Joe Groff via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>> 
>>>>> On Jan 28, 2016, at 2:36 PM, Jacob Bandes-Storch <jtbandes at gmail.com <mailto:jtbandes at gmail.com>> wrote:
>>>>> 
>>>>> I like this idea, but the syntax seems dangerously close to a call site for  "func *(lhs: Int, rhs: Any.Type)"  (which is obviously ill-advised, but it is allowed).
>>>>> 
>>>>> Maybe we could take advantage of something which would be very invalid under the current grammar, namely (n T) rather than (n * T):
>>>>> 
>>>>>    let values: (4 Int) = (1, 2, 3, 4)
>>>> 
>>>> Sure, or we could lift (4 x Int) from LLVM IR's syntax.
>>> 
>>> How about:
>>> 
>>> 	let values:Int[4] = (1,2,3,4)
>>> 
>>> While it looks a bit like a subscript, it doesn’t make sense in a type declaration at present, so could be a good way to define restrictions of this type (we could even extend it to collections later). If the similarity is too close then:
>>> 
>>> 	let values:(Int[4]) = (1,2,3,4)
>>> 
>>> Could work too? Just some alternatives anyway, as I like the idea.
>> 
>> This kind of syntax doesn't compose well with other type productions. If you parse Int[N][M] naively as (Int[N])[M], then you end up with an array of M (array of N (Int)), which ends up subscripting in the opposite order, array[0..<M][0..<N]. C works around this by flipping the order of multiple array indices in a type declaration, so int [n][m] is really (int [m]) [n], but this doesn't work well for Swift, which has other postfix type productions—how would Int[N]?[M] parse? Choosing a prefix notation for fixed-sized array bounds is better IMO to avoid these pitfalls.
>> 
>> -Joe
>> 
>> 
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> 
> 
> -- 
> Trent Nadeau

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